VARIATION OF RAINFALL WITH LATITUDE AND ALTITUDE

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Preston Webster
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1 CHPTER 5 VRITION OF RINFLL WITH LTITUDE ND LTITUDE

2 5.1 GENERL Kerala is an elongated coastal State lies in the south-western part of India. It lies between latitudes 8 N and 13 N comprising approximately 560 km in the north-south direction. Physiographically, Kerala extends from the lowlands adjoining the western sea bed, the landscape ascends steadily towards the east to the mid-lands and further on to the high lands sloping down from the Western Ghats. The mountain ranges which form a natural wall separating Kerala from the adjoining states, have an average elevation ot about 1 km with peaks rising to over 2 km. The namalai and the Nilgiris are the tallest mountains in the Western Ghats skirting the west-coast of Indian peninsula. remarkable feature is the Palghat Gap of about 25 km width between the namalai and the Nilgiris which is the only marked break in the mountain wall. The western slopes of the hills and mountains at the eastern border of Kerala receive copious raintall during the monsoon season. The nature of rainfall variations from south to north of Kerala associated with changes in latitude have been studied and are reported in the present chapter. For the purpose ot the study, the variations in rainfall amount and the number of rainy days with latitude during annual as well as southwest and northeast monsoon periods were studied 139

3 separately. Similarly, the variability ot raintall with stations located at different heights above mean sea level were also carried out to find the influence of altitude on the changes in rainfall pattern of the State. 5.2 DT Monthly mean raintall data from 84 stations distributed in the entire State were utilised for the study. The stations are located latitude-wise from Parassala (8 0 20' N) in the south to Kasargod ( ' N) in the north, and longitude-wise from the western-most station, Kasargode ( ' E) which is also the northernmost station- to the eastern-most station, Marayur ( 'E). Table 4.1 lists the stations selected, their geographical location, altitude above mean sea level, number of years of data used, number ot rainy days and the climatological monthly and annual rainfall data. 5.3 SPTIL DISTRIBUTION OF NNUL RINFLL three-dimensional representation of the latitude-longitude distribution of annual rainfall over Kerala State is depicted in Fig nnual rainfall showed two distinct peaks in the south-central as well as northcentral regions. The southern peak is located in IdukJci District and the northern peak is noted in Kozhikode 140

4 Fig Spatial distribution of annual rainfall 141

5 District. region ot low raintall can be seen between the above two peaks is due to the Jow raintall zone in tile Palghat Gap located around 11 N. 5.4 LTITUDINL VRITION OF NNUL RINFLL Fig is a plot of the climatological mean annual rainfall with latitudes from 84 evenly distributed rainfall stations over Kerala, located trom southernmost part to the northern latitude. The curve of best-fit of rainfall against latitude is obtained by using the annual 6-degree polynomial equations and are drawn as shown in Fig From both Figs. 5.1 and 5.2, it can be seen that the annual rainfall over Kerala show an increase from southern-most latitude of Kerala to about 120 km northwards. Parassala, the southern-most rainfall station, located at latitude 8 20' N, experiences the lowest annual mean rainfall of 1479 mm, which steadily increase to 3400 mm at about 9 30' N. Between latitudes 9 30' Nand 10 IS' N, annual rainfall showed larger spatial variability. Three stations in this latitude belt showed annual rainfall well below 2000 mm. These stations are Santhanpara (1861 mm), Kumily (1721 mm) and Marayur (1349 mm). Even though the above stations are located at high altitude regions, annual rainfall is quite low comparing to other stations in the same latitude region. The main reason tor a decrease in raihtall 142

6 7000 'E 6OOO E '-" Lt z 0::: z 1000 a LTITUDE 13 Fig. 5.2 Scatter diagram of annual rainfall versus latitude 143

7 in these stations are due to their location on the leeward side of the mountain, which reduces rainfall. On the other hand, stations which are located on the same latitude zone, namely Pirmed (4471 mm), Neriamangalam (4576 mm) and Kanjirapally (4151 mm) experiences very heavy rainfall, well above 4000 mm annually. The above high rainfall regions are not located at the same altitude levels. Neriamangalam is situated almost nearer to mean sea level, only at an altitude of 45 gpm from mean sea level. Kanjirappally is located at an elevation of 400 gpm and Pirmed is at 950 gpm. Higher amount of rainfall recorded at these stations are due to their position on the windward side of the mountain which favours condition for good rainfall. fter attaining the peak around 9 30'N, the annual rainfall shows a decreasing trend with latitude to another 150 km northwards. The Palghat Gap plays a major role in reducing the rainfall at 11 N. n increasing trend in raintall with large variability is again seen from 11 N to about 12 N. Heavy rainfall pockets of Kerala are situated around 12 N latitude region. Mavattom, is located at 11 40' N, recorded the highest annual rainfall region of Kerala. In this station, an annual rainfall ot 6073 mm with an average ot 136 rainy days per year is reported. In addition, three more stations experiences annual raintall well over 4000 mm in this latitude zone. They are Karingad (4918 mm), Vythjri 144

8 (443 mm) and Kuttiadi (4504 mm). Northern portion of Kerala, beyond 12 N show a slight decrease in raintall with lower variability. In general, annual rainfall of Kerala shows an increase from 1479 mm in the extreme south latitude (8 20' N) at Parassala to over 3500 mm in the northern-most latitude (12 31' N) at Kasaragod. There are two heavy raintall pockets seen over Kerala. One of the heavy rainfall zone is located in the southern halt ot the State, located between latitudes 9 and 10 N. Kanjirappally, Neriamangalam and Pirmed stations are located in this zone. Northern heavy raintall pocket is situated in the district of Kozhikode, located near 12 N latitude. Heavy rainfall stations situated in this latitude region are Mavattom, Vythiri, Kuttiadi and Karingad. The rainfall variation in the coastal zone of Kerala shows that the coastal belt trom Cochin to Kozhikode receives an annual rainfall of around 3000 mm. The rainfall exhibits a sharp increase to the north ot Kozhikode and a decrease to the south of Cochin along the coastal belt. There is a rapid decrease in raintall is seen trom Kayamkulam to the southern part ot Kerala. 145

9 5 VRI'I'ION OF 'I'HE NUMBER OF NNUL RINY DYS Number of annual rainy days versus latitude is plotted in Fig. 5.3 and the best-tit curve is drawn using the 6-degree polynomial equations. There is a sharp increase in the number of rainy days with increase in latitude is noted in the southern part of Kerala, and attained the peak ot 165 days near 9 45' N latitude. Thereafter, the number ot annual rainy days decrease with latitude. The annual number ot rainy days attained the secondary minimum around 11 N, the region near the Palghat Gap, and then showed an increasing trend, reached the second maximum around 12 N. The rainy days decrease with latitude thereafter. Lowest number of annual rainy days (84) reported at Parassala, the southernmost latitude station of Kerala. nother station Marayur, located near to the Tamil Nadu border and also on the leeward side ot the Western Ghats, has lower number of rainy days in an year. On the other hand, highest number of annual rainy days are seen in the central Kerala, situated in Idukki District. The Kanjirapally station, located at latitude 9 34' N, recorded 163 number of annual rainy days, and Karikode 9 50' N, has 160 annual rainy days. comparison between Fig. 5.2 and Fig. 5.3 explain that the area of peak raintall is not always associated with the rgion of highest number ot rainy days. From Fig. 5.2 it 146

10 (/) 0 z >- «a:::: , a LTITUDE Fig. 5.3 Scatter diagram of annual number of rainy days against latitude 147

11 is evident that the highest annual rainfall region is located in northern Kerala, situated in Kozhikode District. Fig. 5.3 show that the maximum number ot annual rainy days are located in the central Kerala, between go and 100 N latitude belt. The best-tit curve in both Figs 5.2 and 5.3 showed two peaks. The annual rainfall curve showed the maximum peak at higher latitude zone than the lower latitude, whereas the annual number ot rainy days curve showed its highest peak in the central part of Kerala rather than in the northern latitudes. It is evident trom these results that southern Kerala experiences more rainy days with lower intensity of rainfall. On the other hand, northern Kerala gets higher amount of rainfall with relatively less number of rainy days. The duration as well as the intensity of raintall is quite large in the northern part of Kerala. The study is further extended to find the influence of latitud on the two major rainy seasons of Kerala, viz., southwest monsoon season (June - September) and the northeast monsoon season (October - December), separately. 5.6 SPTIL DISTRIBUTION OF SOUTHWEST MONSOON RINFLL Fig. 5.4 is the three dimensional representation ot the latitude-longitude distribution of southwest monsoon raintall over the entire State. It is evident trom Fig. 5.4 that the raintall during the southwest monsoon season have an 148

12 Fig Dirnensional representation of SW monsoon rainfall over Kerala 149

13 increasing trend tram the southern part ot Kerala to the North. Southwest monsoon raintall over Kerala region showed an increasing tendency with latitude. Superimposed on this increasing tendency, there are two distinct regions where the southwest monsoon rainfall peaks, one is situated in Idukki District and the other in Kozhikode District. Both these heavy rainfall pockets are seen in annual rainfall variations. In middle Kerala, the rainfall showed a decrease which is associated with the Palghat Gap. 5.7 LTITUDINL VRITION OF SOUTHWEST MONSOON RINFLL Fig. 5.5 shows the scattergram of the total rainfall during the southwest monsoon period, i.e., June to September, versus latitude over Kerala region. The relationship between the southwest monsoon raintall and latitude can be represented with a linear regression equation and the line of best fit is drawn as illustrated in Fig linear increase in the summer monsoon rainfall is noted trom south to north of Kcrala. Southwest monsoon rainfall is very less in the southern part of Kerala, ot the order ot 1000 mm during this season and increase four times of the order ot 4000 mm in the northern latitude of the State. It is evident trom the scattergram (Pig. 5.5) that relatively large variability in southwest monsoon rainfall is noted between 10 and 12 N, where the position and the orientation of the orography controls the rainfall activity. Northern Kerala 150

14 5000..,,--...,4000 E E '--'3000 -' -' <{ LL 2000 Z <t: er Fig. 5.5 Scatter plot of southwest monsoon rainfall versus latitude 151

15 experienced raintall on the higher side ot the regression line ot the scattergram. The linear regression equatiop obtained trom the latitudinal distribution of southwest monsoon raintall over Kerala shows a positive slope and derives an increase in rainfall of the order ot 464 mm for every 1 latitude distance. In other words, assuming the latitudinal distance is approximately 110 km, then the increase in rainfall during the southwest monsoon season is of the order of 4.2 mm, /kilometer distance. Computed correlation coetficient between southwest monsoon rainfall and the latitude is 0.69'1 tor the 84 stations considered for the study reveal that the computed correlation coefficient (see Table 5.1) is highly significant of the order ot >99% level of signiticance with 82 degrees of freedom. Table 5.1 Correlation and regression coetticients between latitude and rainfall/rainy days Rainfall Rainy days Corr. coeff. Reg. coeff. (mm/olat) Corr. coeff. Reg. coeft. (days/olat) nnual Southwest Monsoon Northeast Monsoon , 0.69'/ '/

16 5.7.1 Number ot Rainy Days in Southwest Monsoon s the case of the amount ot rainfall during the southwest monsoon season, the number of rainy days during the period also showed a linear increase with latitude over Kerala. scattergram plotted between the number of rainy days and latitude and the linear regression line are shown in Fig The number of southwest monsoon rainy days are less in the southern part of Kerala which increases towards the northern part of the State. The number of rainy days in southwest monsoon period also showed a positive correlation with latitude. The correlation coefficient computed between the number of rainy days and latitude during the summer monsoon season gives a positive and statistically signiticant correlation ot The regression coefficient show that the number ot rainy days during the southwest monsoon increases of the order of 6.45 days per latitude from the south of Kerala to the north. S.8 SPTIL DISTRIBUTION OF NORTHEST MONSOON RINFLL Spatial variation in raintall, represented by the 3-dimensional graphical presentation of rainfall during the northeast monsoon season is depicted in Fig.5.? The decrease in raintall trom south to north ot Kerala can be seen in the 153

17 U >- 80 Z «er: 1.L 60 0 er W m 40 ::2 => Z " JI If.. l 11.. b_ tft!- " d " Fig. 5.6 Scatter plot of normal SW monsoon rainy days against latitude 154

18 Fig..1. part trom localised high raintall zones noted in the distribution, the rainfalj during the northeast monsoon season have a strong inverse dependence on latitude. 5.9 LTITUDINL VRITION OF NORTHEST MONSOON Latitudinal variation in northeast monsoon rainfall over Kerala observed during the season October to December is plotted in the scattergram and the linear regression line is drawn as shown in Fig The rainfall during the northeast monsoon season is found to be decreasing with increasing latitude, which is just opposite to that observed in the case of southwest monsoon season. Higher rainfall during the months of October to December is noted in the southern part of Kerala and it decreases towards the northern latitudes. Relatively higher rainfall is noted between 9 and 10 N latitude in the SOllthern half of Kerala and around 12 N in the northern part of the State. Rainfall in both these regions, showed higher values than the regression line drawn in the scatter diagram. In the southern region, the mean rainfall during the northeast monsoon period is estimated as 700 mm, which decreased to its half of 350 mm in the northern part of Kerala. The regression coefficient between rainfall and latitude showed that the rainfall during the northeast monsoon season is decreasing at the rate of 79 mm per degree 155

19 "'. '" Fig Dirnensional representation of NE monsaon rainfall over Kerala 156

20 1000,, E.... E.. '---" 600.t\a It Z...,s 6 4 -«et: 200 Fig.5.B Northeast monsoon rainfall versus latitude 157

21 latitude. The computed correlation coetticient between latitude and raintall for the 84 stations considered for the study showed a value ot , which is negatively signiticant at 99% level ot significance Number of Rainy Days in Northeast Monsoon verage number of rainy days during the northeast monsoon season versus latitude is plotted in Fig s in the case of rainfall during this season, the number ot rainy days are also decreasing with latitude. South ot 9 N latitude, average number of rainy days are less to that of the regression line. Higher number of rainy days are noted between 9 and 10 N latitude region in the southern part and around 12 N in the northern part of Kerala. Both these regions showed higher raintall than that estimated by the regression equation. The region located between 10 and 11 30' N latitude belt experiences lower number of rainy days during south this season. The average number ot rainy days in is estimated as 32 is decreased to its half in the the northern region. The estimated correlation coefficient between latitude and number ot rainy days during the northeast monsoon season is , which is also highly significant. The regression coefficient between the above two parameters showed a value ot rainy days per degree latitude. 158

22 40 35 (f) 30 o >- 25 z & x )( X X )( XX 11 x xx 10L-----L-----L LTITUDE Fig. 5.9 Scatter plot of NE monsoon rainfall days against latitude 159

23 5.10 SPTIL DISTRIBUTION OF STTION HEIGHT Fig represents the height ot stations above mean sea level considered for the study and their geographical location, which is represented in a 3- dimensional graphical manner using the SURFER graphics software. High altitude stations are generally situated in the south-central part of the State. High range stations like Munnar (1150 m), Devikulam (1650 m), Kumily (1100 m) and Santanpara (1097 m) are all situated in this part ot the State. These stations are located above 1 km trom the mean sea level. The location of Palghat Gap can be clearly depicted in the Fig In the northern part of Kerala, high altitude stations, such as mbalavayal (9'l4 m), Vythiri (915 m), Manathavady (900 m) and Chedalath (800 m) are situated. Other stations with lower height above mean sea level can also be seen from the Fig.S.lO EFFECT OF LTITUDE ON RINFLL In this section, a study on the linear relationship between the height of the station above mean sea level and the normal annual rainfall ot the station has been carried out. The study is further extended to find the changes in raintall dl1ring both southwest and northeast monsoon seasons. 160

$, \1 '- 2 t:..j '" '., Fig. 5.$

24 , \1 '- 2 t:..j '" '., Fig Dimensional representation of the height of rainfal l stations above msl

25 The intluence ot the heiqht ot the station above mean sea level on raintall during annual, southwest monsoon and northeast monsoon periods are illustrated in the three scatter diagrams shown in Figs. 5.11, 5.12, and 5.13, respectively. From these figures, it seems that the altitude factor alone does not have signiticant influence on the raintall ot a station. In the case ot annual rainfall, the stations near mean sea level experiences raintall in the range of 500 mm to 3500 mm, in which most of the stations have rainfall around 2000 mm. Stations located at high altitude regions are also recorded rainfall in the same range. It is evident that the criteria of altitude tactor alone does not hold good for relatively higher rainfall compared to that of the mean sea level statjons. Similarly, southwest and northeast monsoon rainfall are also not shown signiticant influence by the station height (see Figs and 5.13). 162

26 Fig Scatter plot of annual rainfall versus altitude

27 Fig Scatter diagram of SW monsoon rainfall vs altitude

28 t-' '" Ul E CJl '--" 0 w 1010 Cl.- => <! 510 o 0 o 0 0 <> <> 8 OO 0..Jri I " 1 0 I I I I I I I I I I I I I I I "wtyl I. 0 <> «. <> I I I Id I I I I o RINFLL Fig Scatter diagram of NE monsoon rainfall versus altitude

SUMMARY AND CONCLUSIONS

CHAPTER 8 SUMMARY AND CONCLUSIONS The outcome of the study on the influence of physiographic features, such as orography, longitude, altitude, forest cover, etc., on the latitude, rainfall pattern ot Kerala